default-distrovars: set default KERNEL_IMAGETYPE(S)

default-distrovars: set default KERNEL_IMAGETYPE(S)

With the change from assuming kernels will be named "vmlinuz"
everywhere, to instead using KERNEL_IMAGETYPE, we require that
KERNEL_IMAGETYPE is set to something. Instead of setting the default in
multiple individual files, set it in default-distrovars.inc.

x86(-64) arches get bzImage as the default. Others get zImage as per
meta/conf/documentation.conf.

Also set KERNEL_IMAGETYPES, as we will eventually be switching away from
KERNEL_IMAGETYPE.

Thanks to Andre McCurdy <armccurdy@gmail.com> for the arch defaults
part.

Signed-off-by: California Sullivan <california.l.sullivan@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>